1. Field of the Invention
The present invention relates to a process for preparing acetoxysilanes by rearing chlorosilanes with acetic acid and acetic anhydride in the presence of a catalyst. More particularly, the present invention relates to the preparation of di-tert-butoxydiacetoxysilane.
2. Description of the Background
Acetoxysilanes have found wide application in the chemical industry. They are suitable, for example, as crosslinking silicon compounds in the production of polymer compositions which can be stored in the absence of water and are curable in the presence of moisture even at room temperature. Examples are compounds such as methyl-, ethyl- and propyltriacetoxysilane.
It is known that organoalkoxysilanes can be prepared by reacting the corresponding chlorosilanes with anhydrous sodium acetate, with acetic anhydride or with acetic acid. The yield of this reaction can also be increased if it is conducted in the presence of a tertiary base. Furthermore, tetreacetoxysilane can also be obtained from silicon tetrachloride and acetic acid in a low-boiling organic solvent, e.g. ether [W. Noll, Chemie und Technologie der Silicone, VCH Weinheim, pages 78 and 79 (1960); Inorganic Syntheses Vol. IV, pages 45-47 (1953)].
U.S. Pat. No. 2,566,347 describes how carbonoyloxysilanes can be prepared under controlled conditions from a halosilane and an organic carboxylic acid in liquid organic solvents such as pentane, ethyl bromide, isopropyl ether, benzene and carbon tetrachloride. Tetracetoxysilane can be converted, for example, into a dialkoxydiacetoxysilane by reaction with a corresponding amount of an alcohol.
FR 1 003 073 discloses the batchwise and simultaneous preparation of carbonoyloxysilanes and carboxylic acid chlorides by reacting organochlorosilanes with monocarboxylic anhydrides.
EP 0 845 469 teaches a process for preparing organocarbonoyloxysilanes by a catalyzed reaction of organochlorosilanes with a carboxylic acid and a carboxylic anhydride in two reaction steps. It is found that secondary reactions, for example, the thermal decomposition of the product formed or a reaction of HCl with acetic acid, can occur to an increased extent in the absence of a solvent, which leads to a significant decrease in yield.
One embodiment of said process is conducted using hexane as solvent.
In the first reaction step, acetic acid is added to an initially charged mixture of alkylchlorosilane and hexane at the boiling point, with hydrogen chloride being evolved.
In the second step, the resulting reaction mixture is added to excess acetic anhydride. Acetyl chloride is formed as a product of the reaction and it and hexane are removed by distillation. After removal of excess acetic anhydride, the crude acetoxysilane remains.
The acetyl chloride/hexane mixture obtained can be hydrolyzed with the equivalent amount of water to give an acetic acid/hexane mixture, which is subsequently separated by distillation and then returned to the synthesis.
As solvents or diluents, EP 0 845 469 also discloses pentane, benzene, toluene and trichloroethylene.
However, the following comments may be made on this intrinsically advantageous process:
In order to avoid introducing free water into the system, the acetyl chloride/hexane hydrolysis has to be conducted very precisely. This procedure is equivalent to a titration which is costly and often subject to errors on an industrial scale. This is because water in the system produces siloxanes on contact with silanes, thereby reducing the quality of the product and also the yield.
After the reaction with water, the resulting acetic acid/hexane mixture has to be separated by distillation. This distillation is likewise costly and requires, among other things because of the high melting point of acetic acid, a complicated condensation system which is generally susceptible to malfunction.
In the first reaction step and in the acetyl chloride/hexane work-up, hydrogen chloride is discharged from a hexane-containing solution. This discharge presents the problem that considerable amounts of hexane can be passed into the waste gas handling system, where it is, for example, carried by inert gases and thus discharged into the atmosphere. A need, therefore, continues to exist for improvements in the synthesis of acetoxysilanes in good yields and purity.